1. Field of the Invention
This invention relates to a surface-condition inspection apparatus, and more particularly, to a surface-condition inspection apparatus which is suitable for detecting a foreign particle, such as dust or the like, adhered to a surface of a photomask or a reticle (hereinafter generically termed a reticle) used in a semiconductor production process by irradiating the surface with a scanning light beam.
2. Description of the Related Art
In the IC (integrated circuit) production process, a pattern for exposure formed on a reticle is in general transferred onto the surface of a semiconductor wafer coated with a resist using a projection optical system or the like of a semiconductor printing apparatus (a stepper or a mask aligner) to produce an IC.
When a pattern is transferred from a reticle onto the surface of a wafer coated with a resist using a semiconductor printing apparatus, if a defect, such as dust or the like, is on the surface of the reticle, the shape of the defect is also printed in addition to the pattern on the reticle, thus causing a decrease in the yield of IC production.
Particularly when a "stepper" is used which prints a desired reticle pattern a plurality of times by a step-and-repeat process, the shape of one particle of dust on the surface of the reticle is printed on the entire surface of the wafer.
Accordingly, it has become important to precisely detect dust on a reticle. Furthermore, it has also become important to provide a shorter inspection time in order to increase the production yield.
FIG. 1 shows a conventional approach for detecting a foreign particle, such as dust or the like, on a reticle. In FIG. 1, a light beam emitted from a laser light source (not shown) is made to be a scanning beam by a rotating element (not shown), such as a polygon mirror or the like, and is condensed onto a reticle 8 via an f-.theta. lens 5. In order to shorten inspection time, the beam is divided into upper and lower beams by a half-mirror HM. The two beams are guided and condensed onto the upper surface (a blank surface) "a" and the lower surface (a pattern surface) b of the reticle 8 via reflecting mirrors M1 and M2, respectively. The scanning beams scan the respective surfaces of the reticle 8 in a direction orthogonal to the plane of FIG. 1. The reticle 8 is scanned in the direction S.sub.1 .revreaction.S.sub.2 in synchronization with the scanning beams, whereby the entire surfaces of the reticle 8 are inspected. If dust is present on a surface of the reticle 8, scattered light of the beam is generated. The scattered light is first imaged onto a field stop 7a or 7b by a condenser lens 6a or 6b (a cylindrical lens having its generatrix in the scanning direction of the beam, a microlens array or the like). The light passing through the aperture of the field stop is guided to a photomultiplier 9a or 9b via optical fibers 8a or 8b. A detection signal from the photomultiplier is guided to a signal processing system (not shown), which processes the signal together with separately-measured scanning position information, whereby the presence and position of dust on each surface are detected.
The influence of dust adhering to the blank surface and the pattern surface of a reticle on a circuit pattern formed on the reticle will now be described. A circuit pattern be transferred to the surface of a wafer is formed on the pattern surface of a reticle in the form of a single-layer chromium film or a double-layer film made of chromium oxide and chromium. The surface of the wafer and the chromium surface are optically conjugate with respect to a printing lens. Hence, the pattern of small dust particles (having the size of 1-2 .mu.m) adhering to the chromium surface is repeatedly transferred onto the surface of the wafer. On the other hand, dust particles having the same size and adhering to the blank surface are defocused and therefore the shape thereof is not transferred thereto. However, dust particles having a large size (at least 5 .mu.m) obturates part of the illuminating light beam (projected from above the reticle) for the portion of the pattern on the chromium surface situated substantially below the dust, thereby causing unevenness in illuminance. As a result, the total integrated amount of illuminating light differs in accordance with the presence/absence of dust for the same exposure time, thereby causing a change in the line width of the circuit pattern.
The minimum resolution required in such an inspection apparatus for detecting dust is 1-2 .mu.m for the pattern surface, and at least 5 .mu.m for the blank surface.
Recently, the size of reticles has increased. While reticles having a size of 5 inches square and 0.09 inch thick have been mainly used for memories having a capacity of equal to or less than 4M (mega) bits, reticles having a size of 6 inches square and 0.25 inch thick are used for 16M-bit memories in accordance with an increase in the size of the printing picture surface. Such a thick reticle may be reused by polishing and removing about 50 .mu.m of the pattern surface for every reuse, If such an operation is repeated 10 times, the thickness of the reticle is reduced by an amount of about 500 .mu.m.
If such reticles having different thicknesses are randomly introduced in an inspection apparatus, the height of the blank surface changes, as shown in FIG. 2 (for example, the upper surface of the reticle 8 changes from a state indicated by solid lines to a state indicated by broken lines), and the diameter of the incident beam on the blank surface of the reticle thereby changes (In this case, the height of the pattern surface does not change even if the thickness of the reticle changes, since the pattern surface (lower surface) contacts a reticle hand 40 when the reticle is supported on the reticle hand). In an inspection apparatus of this kind, since the intensity of light scattered by a particle is inversely proportional to the square of the diameter of the beam, the above-described fact indicates that inspection sensitivity for the blank surface becomes unstable. For example, when a reticle having standard dimensions is inspected, the diameter (D.sub.0) of the beam protected on the blank surface of the reticle is assumed to be 30 .mu.m at the cross section of the optical axis, and the incident angle (.alpha.) of the beam onto the reticle is assumed to be 30.degree.. If a thin reticle having a thickness difference (.DELTA.d) of 500 .mu.m is introduced, the diameter (D') of the beam on the blank surface becomes 40 .mu.m from the following expression (1). Hence, the amount of scattered light is reduced to 56% in the case of a He--Ne laser light source. That is, if dust particles having the minimum size of 5 .mu.m are usually detected with a diameter of the beam of 30 .mu.m, only dust particles having a size of at least 6.7 .mu.m can be detected for a thin reticle. ##EQU1## (.lambda. is the wavelength of the beam).
To sum up, since the shape of dust on the pattern surface is directly transferred, a severe resolution of about 1 .mu.m is required. The resolution is not influenced by the thickness of the reticle. On the other hand, dust on the blank surface functions to produce unevenness in illuminance, and the required resolution may be as large as about 5 .mu.m. However, the resolution is influenced by the thickness of the reticle.
In spite of the above-described difference between the upper and lower surfaces of the reticle, the same light beam is merely divided and guided onto both the pattern surface and the blank surface in the conventional approach shown in FIG. 1. Hence, the conventional approach has the following disadvantage.
That is, since the diameter of the beam is equal on the pattern surface and the blank surface, and since the angle subtended by the beam incident on the upper surface, 2.theta..sub.BL, is equal to the angle subtended by the beam incident on the lower surface, 2.theta..sub.CR, the diameter of the beam on the blank surface is reduced if the diameter (D.sub.0) of the beam is reduced in order to increase resolution for a particle on the pattern surface. As a result, the rate of change of the diameter of the beam when a change .DELTA.d is produced in the thickness of the reticle also increases from expression (1). This indicates that detection sensitivity changes too much in accordance with a change in the thickness of the reticle.
In order to solve such a problem, a method may be considered wherein the thickness of a reticle is measured in advance, and the focus is readjusted by an amount of a change in the thickness when the blank surface is inspected. This method, however, necessitates a thickness measuring means and a focus adjusting means, thereby causing an increase in the size of the system and in the production cost.